The present invention relates to a device for drilling and for removing material using a laser beam, the device comprising a rotating image rotator, a beam manipulator which, when viewed in the beam direction, is arranged in front of the image rotator and which serves to adjust the angle and position of the beam relative to the rotation axis of the image rotator, and a focusing device located on the output side of the image rotator.
In the car industry, in filtering technology, electronics and many other sectors, small bores or drilled holes are needed for very different applications. Examples are injection nozzles for fuels, in the case of which a large number of drilled holes or bores that are arranged in a definite way ensure a uniform distribution of fuel during the injection process, resulting in reduced fuel consumption. To achieve in this field, and also with other applications, a reproducible distribution that is as homogeneous as possible, the drilled holes must be very small and manufactured with high precision. The typical bore diameters are e.g. in the case of diesel injection nozzles around 100 μm at a material thickness of 1 mm and required accuracies of 1 μm. Other examples with similar demands and partly even smaller bore diameters of 20-50 μm are spinnerets for textile fibers, outlet nozzles for air bearings or starting-hole drillings for wire-cut EDM. In all of these cases classic drilling methods are only used to a limited degree due to the demands made on the material, the aspect ratios and the required bore geometry and machining speed.
Laser technology with its specific radiation characteristics offers an alternative that in the past years led to a great number of applications in the above-mentioned sectors. Different drilling principles are here employed.
In the case of single-shot drilling, a single laser pulse with pulse durations of typically a few 100 μs heats and melts the material and expels it out of the drilled hole by partial evaporation.
In percussion drilling the drilled hole is formed though a number of consecutive pulses. In trepanning a small hole is first produced and the larger bore is then cut out.
In all of these cases the drilling process itself is characterized by a strong melt formation, which results in minor bore quality. The highest qualities are achieved in the so-called helical drilling technique, a planar removal process in which the material is predominantly evaporated with short laser pulses. The individual laser pulses of a highly repetitive laser are set side by side in overlapping fashion and guided along the bore circumference along a circular path. With each complete revolution, depending on laser energy and material, a thin layer of 0.1-10 μm is removed. With a great number of such circular movements the bore proper will then be generated. The bore diameter follows from the circular diameter of the beam rotation and the beam diameter. The degree of overlap of the consecutive pulses is here chosen such that the number of non-irradiated remaining edge portions is as small as possible on the one hand and the laser radiation between two pulses is traveling on to such a sufficient extent on the other hand that it does not fully impinge into the melt bath of the preceding pulse. Typically, the degree of overlap is chosen in a range between 50%-95%.
Since the melt solidifies again after one laser beam travel, the material is removed almost exclusively in vapor form, resulting in high surface qualities of the bore wall and a high reproducibility of the bore. This effect is enhanced by the use of short and ultra-short pulse lasers.
Especially the use of lasers in the femtosecond and picosecond regime permits particularly high bore qualities because the pulse powers are here in the range of 100 MW and the resultant melt film thickness is below 1 μm.
An essential precondition for the use of this drilling process is the rotation of the laser beam on a contour. In the simplest case this is a circular path. Since the circular velocity of the laser beam is extremely high, particularly great demands are made on the optical system for rotating the beam. For instance, the circular velocity is 200 mm/s in the case of a laser beam diameter of 20 μm, an overlap degree of 50% and a pulse frequency of 20 kHz. At a required bore diameter of 60 μm this means rotation frequencies of the laser beam of about 1000 Hz.
These high frequencies can no longer be realized with classic beam deflection systems, for instance galvanometer scanners. For this purpose a number of different systems rotating at fast speeds were developed in the past and already described in the literature.
One possibility of rotating a laser beam on a circular path is offered by an arrangement consisting of rotating wedge plates guiding the laser radiation onto a circular path. In this system the laser radiation is rotating at the same speed as the wedge plates. The setting of bore diameter and widening angle of the bore is here carried out by displacing and rotating the rotating wedge plates relative to one another.
Another possibility is the use of a rotating image rotator through which the laser radiation is passed. After passage through the image rotator the laser radiation is rotating both about the rotation axis of the image rotator and about itself. If a stationary focusing lens is arranged downstream of the image rotator, circular round bores can then be formed by the two rotational movements of the laser radiation that is then focused. Due to the rotation of the laser radiation in itself it is possible to implement even the tiniest bore diameters in that the helical diameter, i.e. the diameter of the laser radiation rotating about the rotation axis of the image rotator, approaches zero. This is however not possible in systems in which the laser radiation is not rotating in itself; this always requires a minimal helical diameter.